1. Field of the Invention
The present invention relates to a substrate holding device capable of holding a plurality of thin substrates, such as semiconductor wafers or glass substrates.
2. Description of the Related Art
A plurality of semiconductor wafers (hereinafter, referred to simply as “wafers”) are stacked in a vertical arrangement in a cassette for carrying. A semiconductor device fabricating apparatus processes a plurality of wafers stacked in a vertical arrangement in a process substrate holder by a predetermined process. A substrate holding device holds a plurality wafers, and carries the plurality of wafers simultaneously between a cassette and the process substrate holder.
In some cases, wafers are stacked at different pitches in a cassette and a process substrate holder, respectively. A substrate holding device disclosed in JP-A No. 2000-174099 intended to deal with such cases adjusts the pitches of wafers on the basis of the respective predetermined initial conditions of the cassette and the process substrate holder by a predetermined adjustment.
FIGS. 16A and 16B are perspective views of process substrate holders 1 and 2. It is possible that the process substrate holder undergoes secular distortion due to exposure to process heat. FIG. 16A shows the process substrate holder 1 in an initial state and FIG. 16B shows a distorted process substrate holder 2 undergone secular distortion.
Referring to FIG. 16A, the new process substrate holder 1 in the initial state is provided with pairs of projections 4a and 4b. The projections 4a and the corresponding projections 4b are in a horizontal plane. The projections 4a and 4b are arranged vertically at equal pitches P in the direction of the arrow Z. Each pair of projections 4a and 4b holds a wafer 3 thereon. Thus the wafers 3 are held horizontally at equal pitches P in the new process substrate holder 1.
When the semiconductor device fabricating apparatus processes the wafers 3 by a hot process, it is possible that the process substrate holder 1 undergoes secular distortion when the process substrate holder is used for a long time in a hot atmosphere. FIG. 17 is a fragmentary sectional view of the process substrate holder 2 undergone secular distortion. The secular distortion of the process substrate holder 2 is exaggerated in FIG. 17 to facilitate understanding. As shown in FIG. 17, in the process substrate holder 2 distorted by aging, vertical intervals D1, D2 and D3 between the projections 4a and 4b have been changed and, consequently, the wafers 3 are arranged at different pitches P1, P2 and P3. For example, even if the wafers 3 are arranged at equal pitches P of 6.3 mm in the new process substrate holder 1, the wafers 3 are arranged at irregular pitches between 6.0 and 6.6 mm in the process substrate holder 2 undergone secular distortion. Although the projections 4a and the corresponding projections 4b are in the same horizontal planes, respectively, in the new process substrate holder 1, the projections 4a and the corresponding projections 4b of the process substrate holder 2 are dislocated vertically relative to each other, respectively, and the levels of the projections 4a and those of the corresponding projections 4b differ from each other by differences D4 and D5, respectively. Consequently, the wafers 3 are held in sloping positions in the process substrate holder 2.
The prior substrate holding device is able to hold wafers 3 only at pitches determined at the introduction of the substrate holding device into a semiconductor device fabricating system. Therefore, the prior substrate holding device is incapable of coping with the irregular change of the pitches of the wafers 3 due to the secular distortion of the process substrate holder and often fails in holding the wafers 3 causing a significant reduction in the yield of the processed wafers 3. The same problem arises when the pitches of the wafers 3 in the cassette are changed.
The prior substrate holding device is incapable of coping with changes in the arrangement of wafers. Although problems that arise when the substrate holding device is applied to holding wafers have been described above, the same problem arise when the substrate holding device is applied to holding a plurality of substrates other than wafers.